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United Security Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Statements of Operations (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Income:                    
Expense                 $ 7,570 $ 7,602
Net income $ 441 $ 544 $ 774 $ 836 $ 720 $ 837 $ 1,218 $ 774 2,595 3,549
United Security Bancshares, Inc. [Member]                    
Income:                    
Dividend income, First US Bank                 1,453 381
Total income                 1,453 381
Expense                 707 372
Gain before equity in undistributed income of subsidiaries                 746 9
Equity in undistributed income of subsidiaries                 1,849 3,540
Net income                 $ 2,595 $ 3,549